Zinc oxide (ZnO) has a wide band gap, high stability and high thermal operating range that make it a suitable material as a semiconductor for fabrication of variety of semiconductor devices. ZnO single crystal is also a substrate material for epitaxial growth and fabrication of gallium nitride (GaN) and III-nitride devices. In order to manufacture these semiconductor devices, the ZnO is provided as large single crystal boules that are further sliced into wafers. The ZnO wafers will then be polished to make ZnO substrates.
A ZnO substrate is used for epitaxial growth of thin films. A thin film grown on a ZnO substrate may be a contiguous thin film, a nano-structure, or a mixture of the two. The materials of thin films that can be grown on ZnO substrates are metal-oxide compounds, including but not limited to, ZnO, CdO, MgO, BeO, ZnxMg1-xO (where 0<x<1), ZnxCd1-xO (where 0<x<1), ZnxBe1-xO (where 0<x<1), and III-nitride compounds, including but not limited to, GaN, InN, AlN, InxGa1-xN (where 0<x<1), AlxGa1-xN (where 0<x<1).
Semiconductor devices that can be made using the thin films of metal oxides or III-nitrides grown on ZnO substrates include, but not limited to, light emitters, such as UV, visible light emitting diodes (LEDs) and laser diodes (LDs), UV photodetectors, high-frequency/RF devices, such as high electron mobility transistors (HEMTs), high-power devices, such as Schottky diodes, PIN diodes, power transistors, high-temperature devices, spintronics devices, radiation detector devices, chemical sensors, surface acoustic devices, and integrated circuits (ICs), etc.
To make semiconductor devices with high performance characteristics, it is important to provide ZnO substrates with low dislocation densities. For some devices, it is desirable to use n-type ZnO substrates with a low electrical resistivity, preferably less than 0.1 ohm-cm, i.e., low resistivity n-type ZnO substrates. For other devices, it is desirable to use p-type ZnO substrates with a low electrical resistivity, preferably less than 0.1 ohm-cm, i.e., low resistivity p-type ZnO substrates. Yet, for some other devices, particularly high-frequency or RF devices, it is desirable to use ZnO substrates with a high electrical resistivity. Therefore, it is desirable that a growth method for producing ZnO single crystal boules is capable of producing all three types of ZnO crystal boules, i.e., n-type low resistivity ZnO crystal boules, p-type low resistivity ZnO crystal boules, and semi-insulating or insulating ZnO crystal boules.
An n-type ZnO single crystal boule with a low resistivity is produced by doping with an n-type dopant (e.g., B, Al, Ga, In, Cl, Br, Ti, etc.) during the crystal growth process. Similarly, a p-type ZnO single crystal boule with low resistivity is produced by doping with a p-type dopant (e.g., P, As, Sb, Na, K, Ag, etc.) during the crystal growth process. A semi-insulating or insulating ZnO crystal boule with a high resistivity can be produced in two ways: (1) maintaining a high purity during the crystal growth process so that a ZnO crystal boule with low concentrations of residual electrically active impurities, and (2) doping ZnO a small amount of an dopant opposite type of the residual impurities or a deep level impurity (usually a transition metal element, such as Sc, V, Fe, etc.) to compensate the residual impurities.
A ZnO single crystal can be unintentionally doped by impurities in a ZnO crystal growth system due to residual impurities in the source materials or contamination from crystal growth furnace components and medium surrounding the growing crystal boule. Therefore, it is desired to maintain a low concentration of impurities in a ZnO crystal growth process.
ZnO bulk single crystals of large sizes (greater than 1 inch in diameter) were demonstrated by using the following three techniques: (1) chemical-assisted vapor transport (CVT) technique, (2) hydrothermal technique and (3) high-pressure melt growth technique. The characteristics of these three techniques are discussed as follows:
CVT Technique
In a CVT growth of ZnO crystals, a ZnO source material in the source zone is reacted with a chemical transport agent (e.g., H2, H2O) and then transported to the growth zone with a ZnO crystal seed so that a ZnO crystal boule is grown. A CVT growth of ZnO crystals is usually carried out in fused silica vessel in a resistively heated growth furnace in the temperature range of 1000-1200° C. An example of CVT growth process for ZnO single crystals is the one developed at Eagle-Picher (later on at ZN Technology, Inc.), and ZnO wafers up to 2-inch in diameter with reasonably good quality were demonstrated.
A drawback of a CVT growth process is the extremely low growth rates (2-3 mm/day, or ˜0.1 mm/hr), which does not facilitate volume production of ZnO single crystals and wafers. In addition, contamination of Si (an n-type dopant in ZnO) from fused silica (SiO2) makes it difficult to produce p-type and semi-insulating ZnO crystals. Finally, it is found that hydrogen is an impurity known to be both an n-type impurity when at a substitutional site and an impurity passivating a p-type dopant by forming a complex with a p-type dopant, thus reducing free carrier concentration and increasing electrical resistivity. As a result, incorporation of hydrogen (H) from the chemical transport agent during CVT growth makes it difficult to produce a reliable and stable p-type and semi-insulating ZnO crystals.
Hydrothermal Technique
Since ZnO is highly soluble in water at high temperatures (300-400° C.), single crystal ZnO can be grown using a hydrothermal growth process, similar to that used to grow quartz single crystals. Hydrothermal growth is a potential contender for commercial volume production of ZnO single crystals for semiconductor wafers. However, there are several drawbacks in hydrothermal growth of ZnO. First, the growth rate is very low, in the range of 0.1-0.2 mm/day (about 0.004-0.008 mm/hr) in the c-axis direction. A (0001) ZnO crystal boule of 20 mm in thickness will take about 100 days to grow, which is considered to be a long cycle time. This problem may be mitigated by scaling up the hydrothermal growth setup to produce large number of single crystals boules per run. The scale-up effort, however, is both lengthy and costly. In addition, a platinum (Pt) metal crucible is used in hydrothermal growth of ZnO, which adds a significant cost to the crystal production.
Another drawback is that hydrothermal-grown ZnO crystals contain large amounts of lithium (Li) and potassium (K) (from LiOH and KOH used in hydrothermal growth solution). These alkaline metal elements are known to be p-type dopants and hence contribute to p-type conductivity, which reduces n-type carrier concentrations and therefore leads to a higher resistivity in an n-type ZnO crystal. Since Li is a fast diffusion ion in a ZnO crystal, out-diffusion of Li from hydrothermal-grown ZnO crystal substrates to the epitaxial thin films during epitaxial growth make it difficult to produce sharp p-n junctions and therefore detrimental to device performances.
A third drawback includes hydrogen (H) contamination in hydrothermal grown ZnO crystals makes it difficult to produce p-type and semi-insulating ZnO crystals, which similar to the situation in CVT growth technique.
Finally, due to the high growth-rate anisotropy inherent in hydrothermal growth technique, it is difficult to grow large-diameter ZnO crystal boules with an orientation other than the [0001] orientation. As a result, it is difficult to produce large diameter ZnO boules in non-polar directions in an efficient manner using a hydrothermal technique.
Melt Growth Technique
Since ZnO melts at 1975 degrees Celsius (° C.), in principle, it can be grown from its melt. Melt growth techniques have many advantages, such as a high growth rate (>5 mm/hr), the flexibility to grow in any crystal orientation, and the ease to dope the crystals to achieve a desired electronic characteristic. However, ZnO sublimes/dissociates at the melting point and the dissociation pressure of ZnO at its melting point is very high (˜1.07 atmospheres) making the melt growth difficult to manage. Therefore, a high-pressure (>20 atm) oxygen gas is required to suppress evaporation and dissociation.
Another difficulty in ZnO melt growth is that only iridium crucible can be used to directly contain ZnO melt. To avoid using a refractory metal crucible to directly contain a ZnO melt, a so-called “skull-melting” (or “cold-crucible”) technique was developed to hold ZnO melt and demonstrated ZnO crystals boules of over 1-inch in diameter. It appears that controlling of ZnO crystal growth using a “skull-melting” technique is challenging because of the complexity of the process. More importantly, seeded growth in a “skull-melting” appears to be difficult. In another development, a Bridgman melt growth of ZnO crystals in an iridium (Ir) metal crucible to contain the ZnO melt at a high pressure under a gas mixture of CO2 and O2 was investigated and ZnO single crystal boules over 1 inch in diameter were demonstrated. Drawbacks of the technique include: (1) the difficulty in controlling the growth process, (2) a severe degradation of Ir crucible, and (3) the contamination of carbon from the gas mixture.
Since ZnO sublimes appreciably at temperatures higher than 1500° C., a physical vapor transport (PVT) growth can be used to grow ZnO single crystals at temperatures higher than 1500° C. A PVT growth is a sublimation and re-condensation process. In U.S. Pat. No. 7,279,040, a PVT growth technique for producing ZnO single crystals at a temperature range of 1300-1800° C. is described. The advantages of a PVT growth for ZnO include the ease to control the growth process and the low cost. However, a PVT growth requires a suitable crucible and a thermal insulation setup. Crucibles made of oxides, such as alumina (a ceramic containing 99.0-99.8% Al2O3) or sapphire (Al2O3), were found to react with Zn vapor at high temperatures, which not only limits the growth temperature to no greater than about 1550° C. but also limits the growth rate to less than 0.2 mm/hr or even lower. In addition, because aluminum (Al) from an Al2O3-based crucible is an n-type dopant in ZnO, a PVT using an Al2O3-based ceramic crucible limits the growth to only n-type ZnO crystals.
There exists a need of a growth technique for producing ZnO single crystals that overcomes the inadequacies of these three techniques discussed above.